Human beings have grown accustomed to controlling their environment. Nature is unpredictable and often presents conditions that are far from a human being's ideal living conditions. The human race has therefore tried for years to engineer the environment inside a structure to emulate the outside environment at a perfect set of conditions. This has involved temperature control, air quality control and lighting control.
The desire to control the properties of light in an artificial environment is easy to understand. Humans are primarily visual creatures with much of our communication being done visually. We can identify friends and loved ones based on primarily visual cues and we communicate through many visual mediums, such as this printed page. At the same time, the human eye requires light to see by and our eyes (unlike those of some other creatures) are particularly sensitive to color.
With today's ever-increasing work hours and time constraints, less and less of the day is being spent by the average human outside in natural sunlight. In addition, humans spend about a third of their lives asleep, and as the economy increases to 24/7/365, many employees no longer have the luxury of spending their waking hours during daylight. Therefore, most of an average human's life is spent inside, illuminated by manmade sources of light.
Visible light is a collection of electromagnetic waves (electromagnetic radiation) of different frequencies, each wavelength of which represents a particular “color” of the light spectrum. Visible light is generally thought to comprise those light waves with wavelength between about 400 nm and about 700 nm. Each of the wavelengths within this spectrum comprises a distinct color of light from deep blue/purple at around 400 nm to dark red at around 700 nm. Mixing these colors of light produces additional colors of light. The distinctive color of a neon sign results from a number of discrete wavelengths of light. These wavelengths combine additively to produce the resulting wave or spectrum that makes up a color. One such color is white light.
Because of the importance of white light, and since white light is the mixing of multiple wavelengths of light, there have arisen multiple techniques for characterization of white light that relate to how human beings interpret a particular white light. The first of these is the use of color temperature, which relates to the color of the light within white. Correlated color temperature is characterized in color reproduction fields according to the temperature in degrees Kelvin (K) of a black body radiator that radiates the same color light as the light in question. FIG. 1 is a chromaticity diagram in which Planckian locus (or black body locus, or black body line, or white line) (104) gives the temperatures of whites from about 700 K (generally considered the first visible to the human eye) to essentially the terminal point. The color temperature of viewing light depends on the color content of the viewing light as shown by white line (104). Thus, early morning daylight has a color temperature of about 3,000 K while overcast midday skies have a white color temperature of about 10,000 K. A fire has a color temperature of about 1,800 K and an incandescent bulb about 2848 K. A color image viewed at 3,000 K will have a relatively reddish tone, whereas the same color image viewed at 10,000 K will have a relatively bluish tone. All of this light is called “white,” but it has varying spectral content.
The second classification of white light involves its quality. In 1965 the Commission Internationale de l'Eclairage (CIE) recommended a method for measuring the color rendering properties of light sources based on a test color sample method. This method has been updated and is described in the CIE 13.3-1995 technical report “Method of Measuring and Specifying Colour Rendering Properties of Light Sources,” the disclosure of which is herein incorporated by reference. In essence, this method involves the spectroradiometric measurement of the light source under test. This data is multiplied by the reflectance spectrums of eight color samples. The resulting spectrums are converted to tristimulus values based on the CIE 1931 standard observer. The shift of these values with respect to a reference light are determined for the uniform color space (UCS) recommended in 1960 by the CIE. The average of the eight color shifts is calculated to generate the General Color Rendering Index, known as CRI. Within these calculations the CRI is scaled so that a perfect score equals 100, where perfect would be using a source spectrally equal to the reference source (often sunlight or fill spectrum white light). For example a tungsten-halogen source compared to full spectrum white light might have a CPU of 99 while a warm white fluorescent lamp would have a CRI of 50.
Artificial lighting generally uses the standard CRI to determine the quality of white light. If a light yields a high CRI compared to full spectrum white light then it is considered to generate better quality white light (light that is more “natural” and enables colored surfaces to be better rendered). This method has been used since 1965 as a point of comparison for all different types of light sources.
In addition to white light, the ability to generate specific colors of light is also highly sought after. Because of humans' light sensitivity, visual arts and similar professions desire colored light that is specifiable and reproducible. Elementary film study classes teach that a movie-goer has been trained that light which is generally more orange or red signifies the morning, while light that is generally more blue signifies a night or evening. We have also been trained that sunlight filtered through water has a certain color, while sunlight filtered through glass has a different color. For all these reasons it is desirable for those involved in visual arts to be able to produce exact colors of light, and to be able to reproduce them later.
Current lighting technology makes such adjustment and control difficult, because common sources of light, such as halogen, incandescent, and fluorescent sources, generate light of a fixed color temperature and spectrum. Further, altering the color temperature or spectrum will usually alter other lighting variables in an undesirable way. For example, increasing the voltage applied to an incandescent light may raise the color temperature of the resulting light, but also results in an overall increase in brightness. In the same way, placing a deep blue filter in front of a white halogen lamp will dramatically decrease the overall brightness of the light. The filter itself will also get quite hot (and potentially melt) as it absorbs a large percentage of the light energy from the white light.
Moreover, achieving certain color conditions with incandescent sources can be difficult or impossible as the desired color may cause the filament to rapidly burn out. For fluorescent lighting sources, the color temperature is controlled by the composition of the phosphor, which may vary from bulb to bulb but cannot typically be altered for a given bulb. Thus, modulating color temperature of light is a complex procedure that is often avoided in scenarios where such adjustment may be beneficial.
In artificial lighting, control over the range of colors that can be produced by a lighting fixture is desirable. Many lighting fixtures known in the art can only produce a single color of light instead of range of colors. That color may vary across lighting fixtures (for instance a fluorescent lighting fixture produces a different color of light than a sodium vapor lamp). The use of filters on a lighting fixture does not enable a lighting fixture to produce a range of colors, it merely allows a lighting fixture to produce its single color, which is then partially absorbed and partially transmitted by the filter. Once the filter is placed, the fixture can only produce a single (now different) color of light, but cannot produce a range of colors.
In control of artificial lighting, it is further desirable to be able to specify a point within the range of color producible by a lighting fixture that will be the point of highest intensity. Even on current technology lighting fixtures whose colors can be altered, the point of maximum intensity cannot be specified by the user, but is usually determined by unalterable physical characteristics of the fixture. Thus, an incandescent light fixture can produce a range of colors, but the intensity necessarily increases as the color temperature increases which does not enable control of the color at the point of maximum intensity. Filters further lack control of the point of maximum intensity, as the point of maximum intensity of a lighting fixture will be the unfiltered color (any filter absorbs some of the intensity).